Optical disc, and reading system and manufacturing method of the disc

ABSTRACT

A writable optical disc has an information writing track, and a guiding track for introducing a laser beam to the information writing track. The guiding track has prepit information recorded thereon, the prepit information including at least address information of the optical disc. The prepit information is recorded on the guiding track at a position where interference of adjacent guiding tracks with the prepit information is prevented. The information writing track is a groove track, and the guiding track is a land track.

This is a division of application Ser. No. 08/668,334 filed Jun. 25,1996 now U.S. Pat. No. 6,181,657. The disclosure of the priorapplication(s) is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates to a writable optical disc, and a readingsystem and a manufacturing method of the disc.

In a writable optical disc, information such as a synchronizing signaland address information (hereinafter called prepit information) forsearching a position on the disc are previously recorded by lasercutting at a preformat stage in a manufacturing process. As one ofmethods of the preformat of the prepit information, a track (groove orland) on which information is written by a user is wobbled. In anothermethod, prepits are recorded on the track.

However, in the former method, the track itself is wobbled in a waveformin right and left by a wobbling signal. Therefore, the magnitude of themodulation by the wobbling signal is limited to a small degree. As aresult, the C/N decreases. Furthermore, since the interference of theadjacent tracks at the wobbling must be considered, pitch between thetracks can not be reduced. Consequently, the writing capacity of thedisc is limited.

In the latter method, since the prepits are recorded, the space forwriting the information is reduced. Therefore, the available efficiencyof the disc is low.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a writable disc inwhich the writing capacity of the disc is increased.

Another object of the present invention is to provide a reading systemof the disc which can be effectively read out written information.

A further object of the present invention is to provide a manufacturingmethod of the disc where the disc is efficiently manufactured.

According to the present invention, there is provided a writable opticaldisc having an information writing track, and a guiding track forintroducing a laser beam to the information writing track, characterizedin that the guiding track has prepit information recorded thereon, theprepit information including at least address information of the opticaldisc.

The prepit information is recorded on the guiding track at a positionwhere interference of adjacent guiding tracks with the prepitinformation is prevented. The information writing track is a groovetrack, and the guiding track is a land track.

In accordance with the present invention, a reading system for reading awritable optical disc having an information writing track, and a guidingtrack for introducing a laser beam to the information writing track isfurther provided. The reading system comprises a photodetector havingtwo photodetecting elements divided by a first line optically parallelwith a radial direction of the disc for detecting reflected light of alaser beam reflected from the information writing track, and prepitinformation extracting means for extracting prepit information recordedon the guiding track based on a difference signal between outputs of thedivided two photodetecting elements.

The photodetector is further divided by a second line optically parallelwith a tangential direction of the information writing track of theoptical disc, and the difference signal is obtained from an output of apair of photodetecting elements positioned on an inside portion of thesecond line and an output of a pair of photodetecting elementspositioned on an outside portion of the second line for producing atracking error signal.

Another reading system for reading a writable optical disc having aninformation writing track, and a guiding track for introducing a laserbeam to the information writing track comprises a first photodetectorhaving two photodetecting elements divided by a first line opticallyparallel with a tangential direction of the information writing track ofthe disc for detecting reflected light of a first laser beam reflectedfrom the information writing track, a second photodetector having fourphotodetecting elements divided by a second line optically parallel withthe tangential direction of the information writing track of the disc,and divided by a third line optically parallel with a radial directionof the disc for detecting reflected light of a second laser beamreflected from the guiding track, first difference signal producingmeans for producing a first difference signal based on a differencebetween outputs of the divided two photodetecting elements of the firstphotodetector, second difference signal producing means for producing asecond difference signal based on a difference between outputs of a pairof photodetecting elements positioned at an inside portion and at anoutside portion of the second line of the second photodetector, thirddifference signal producing means for producing a third differencesignal based on a difference between outputs of a pair of photodetectingelements positioned on a portion at a reading forward direction side andat a reverse direction side to the reading forward direction of thethird line of the second photodetector, tracking error producing meansfor producing a tracking error signal based on the first differencesignal and the second difference signal, and prepit informationextracting means for extracting prepit information on the guiding trackbased on the third difference signal.

The present invention provide a reading system for reading a writableoptical disc having an information writing track, and a guiding trackfor introducing a laser beam to the information writing track whereinnoises included in the prepit information is removed. The reading systemcomprises a photodetector having two photodetecting elements divided bya line optically parallel with a radial direction of the disc fordetecting reflected light of a laser beam reflected from the informationwriting track, prepit signal producing means for producing a differencesignal as a first prepit signal, from outputs of the dividedphotodetecting elements, added signal producing means for producing anadded signal of outputs of the divided photodetecting elements, noisecancel signal producing means for producing a noise cancel signal basedon the added signal, and noise extracting means for subtracting thenoise cancel signal from the first prepit signal to produce a secondprepit signal without noises caused by the added signal.

Another reading system for reading a writable optical disc having aninformation writing track, and a guiding track for introducing a laserbeam to the information writing track comprises a photodetector havingtwo photodetecting elements divided by a line optically parallel with aradial direction of the disc for detecting reflected light of a laserbeam reflected from the information writing track, prepit signalproducing means for producing a difference signal as a first prepitsignal, from outputs of the divided photodetecting elements, noisecancel signal producing means for producing a noise cancel signal basedon a laser beam for writing information on the information writingtrack, and noise extracting means for subtracting the noise cancelsignal from the first prepit signal to produce a second prepit signalwithout noises caused by the writing laser beam.

There is further provided a method for manufacturing a writable opticaldisc having an information writing groove, a guiding land, and prepitinformation recorded on the land, the method comprising steps ofperforming a laser cutting to expose a resist coated on an originalglass substrate by a laser beam modulated based on prepit information tobe recorded on a land, developing the exposed resist, thereby forming aland and prepits on the land, performing electroforming to the developedglass substrate for producing a master stamper, performingelectroforming to the master stamper for producing a sub stamper, andreplicating an optical disc by using the sub stamper.

These and other objects and features of the present invention willbecome more apparent from the following detailed description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged perspective view partly showing an optical disc ofthe present invention;

FIGS. 2a and 2 b are schematic diagrams showing examples of prepitsformed on the disc;

FIG. 3 is a diagram showing a manufacturing process of the disc;

FIG. 4 is a diagram showing a cutting machine of the disc using a laserbeam;

FIGS. 5a and 5 b are diagrams showing a reading system to which thepresent invention is applied;

FIG. 6 is a diagram showing signals read out by the reading system withrespect to the conditions of the disc;

FIG. 7 is a diagram showing output characteristics of a push-pull signaland an RF signal with respect to depth of a groove;

FIGS. 8a and 8 b are diagrams showing a reading system of a secondembodiment of the present invention;

FIGS. 9a and 9 b are diagrams showing a reading system of a thirdembodiment of the present invention;

FIG. 10 is an explanatory diagram showing signals for producing a groovepit canceling signal of the third embodiment;

FIGS. 11a and 11 b are diagrams showing a reading system of a fourthembodiment of the present invention;

FIG. 12 is a diagram showing power of a laser beam; and

FIG. 13 is an explanatory diagram showing signals for producing a groovepit canceling signal of the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 typically showing a writable optical disc of thepresent invention, an optical disc 1 comprises a transparent layer 1 amade of transparent resin such as polycarbonate, a coloring matter 1 bunder the transparent layer 1 a, and a metallic reflection film 5. Onthe underside of the transparent layer 1 a, a plurality of grooves 2 (asviewed from the reflection film 5) as information writing tracks, and aland 3 as a guiding track are spirally formed from a central portion ofthe disc to an outer periphery thereof. Prepits 4 are recorded on theland 3 as the guiding track for prepit information by the preformat. Themetallic reflection film 5 is coated on the coloring matter 1 b. Aprotection film (not shown) is layered on the reflection film 5. In FIG.1, a laser beam is projected from a portion above the disc 1 to thegroove 2 and the land 3.

FIG. 2a shows the track of the disc 1 which is spirally formed thereon.The prepits 4 are recorded on every other land 3 for preventing theprepit information from interfering. When writing or reproducing theinformation by a pickup, if the prepits 4 are recorded on each of thelands 3, the pickup may read information of prepits 4 recorded onadjacent lands which are disposed opposite sides of the groove 2 at thesame time. Accordingly, the prepit information can not be ensurelyreproduced.

FIG. 2b show another example tracks for preventing the interference ofthe information. A recording pattern comprising a first pattern and asecond pattern is provided for recording the prepit information on theland. Each of the first and second patterns has sync Sy (at every SYNCframe) and ID information necessary for servo. The sync Sy and IDinformation recorded by the first pattern are deflected with apredetermined phase difference, for example, with 180 phase differencefrom the sync Sy and ID information recorded by the second pattern.

The prepit information is first recorded in the spiral direction by thefirst pattern. If the sync Sy and the ID of the adjacent lands arealmost overlapped, the first pattern is changed to the second pattern tocontinue recording. Then, if the sync Sy and the ID of the adjacentlands of the second pattern are almost overlapped, the first pattern isused again.

FIG. 3 shows the manufacturing process of the disc.

In a conventional disc, a prepit is recorded on a groove. In cutting ofan original glass substrate, a groove is formed by laser cutting. If theconventional disc is employed for the present invention, the prepit iscut on a land after the groove is cut. Accordingly, the number ofprocesses increases, and it is difficult to precisely cut the prepit.

In the present invention, when a laser cutting is performed, a resistcoated on an original glass substrate is exposed by laser cutting anddeveloped based on the prepit information, thereby forming lands andrecording prepits on the lands at the same time. Thus, a master stamperis made. The master stamper is processed by electroforming to form a submaster, or the sub master is further processed by electroforming at eventimes to form a sub stamper. After the electroforming, a replication ismade to the transparent layer 1 a. Thereafter, the coloring matter 1 bis formed on the underside of the layer 1 a.

In the process, since the substrate is cut by only one cutting process,the prepit is not deflected from the land. Thus, a disc having highaccuracy can be manufactured.

FIG. 4 shows a cutting machine employed with the laser cutting. Thecutting machine comprises a laser 10 for emitting a laser beam having alarge power. An optical modulator 11 is provided for modulating thelaser beam from the laser 10 in accordance with land cutting informationapplied from an encoder 12. A glass substrate 14 having a resist 15coated thereon is mounted on a spindle motor 16. The laser beammodulated by the modulator 11 is focused on the resist 15 by anobjective lens 13 for forming a spot thereon.

The spindle motor 16 is rotated by a rotation detector 17 and a rotationservo circuit 18 at a constant linear velocity (CLV). The motor 16 ismounted on a feeding unit 19 to be moved in a radial direction of theglass substrate 14 by a position detector 20 and a feeding servo circuit21 at a predetermined speed. Thus, the lands and prepits are cut onresist 15 of the substrate 14 in a spiral direction from a centralportion of the substrate to an outer periphery thereof.

FIGS. 5a and 5 b show an embodiment of a reading system for readinginformation written on the disc.

A laser beam for reproducing information is reflected from a prism 32and projected on a recording surface of the disc 1 through an objectivelens 31 to form a spot of the laser beam. The light reflected from thedisc 1 is fed to a photodetector 33 through the prism 32.

FIG. 5b shows the spot of the laser beam on the disc 1. The spot isformed on the groove 2 near the land 3. The photodetector 33 comprisesfour photodetecting elements A, B, C and D divided by a line parallelwith the radius of the disc and a line parallel with the tangentialdirection of the groove 2 as the information recording track.

The divided four elements A to D produce corresponding output signals Ato D which are applied to amplifiers 34 to 37, respectively. A pluralityof adders and subtracters 38 to 43 and 47 are provided for adding andsubtracting amplified signals from the amplifiers. Thus, an RF signal, atracking error signal, and prepit information recorded on the land areread out.

In order to read the information written on the groove 2, all of theoutputs A to D are added.

In order to obtain the RF signal, the outputs A and D are added by theadder 40, and the outputs B and C are added by the adder 41. Outputs(A+D) and (B+C) are added by the adder 43. Thus, the RF signal (A+B+C+D)is produced from a terminal 46.

The tracking error signal is obtained by a difference between theoutputs (A+D) and (B+C) divided in the tracking direction of the groove2. In this state, the difference of a primary light, not a zero levellight, is obtained. The output (A+D) of the adder 40 and the output(B+C) of the adder 41 are subtracted by the subtracter 42. Thus, thetracking error signal (A+D)−(B+C) is produced from a terminal 45.

The prepit information signal (hereinafter called prepit signal) isobtained by a difference between outputs (A+B) and (C+D) divided in theradial direction of the disc. In this state, the difference of a primarylight, not a zero level light, is obtained. The outputs A and B areadded by the adder 38, and the outputs C and D are added by the adder39. The output (A+B) of the adder 38 and the output (C+D) of the adder39 are subtracted by the subtracter 47. Thus, the prepit signal(A+B)−(C+D) is produced from a terminal 44.

As shown in FIG. 2a, when the disc makes one turn, one trackingfinishes, the position of the lands 3 changes from the left (right) tothe right (left) of the groove 2. However, the change of the positioncan be easily detected because polarity of the prepit signal (A+B)−(C+D)produced from the terminal 44 is inverted.

FIG. 6 shows RF, tracking error and prepit signals read out by thereading system with respect to the conditions of the disc.

From the foregoing, it will be seen that signals are sufficiently andexactly read out. The prepit signal does not affect the RF signal,because the depth of the groove of the track is determined as shown inFIG. 7.

FIGS. 8a and 8 b show a second embodiment of the reading system employedwith a three spot method.

As shown in FIG. 8a, three laser beams are projected on the disc 1 toform three spots 51, 52 and 53. The spot 51 is projected on the prepit 4on the land 3. The spot 52 is projected on the groove 2. The spot 53 isprojected on the land 3 without the prepit 4.

As shown in FIG. 8b, the reading system has three photodetectors 54, 55and 56 for detecting the respective spots 51, 52 and 53. Thephotodetector 54 is divided into four photodetecting elements A, B, Cand D in the same manner as the embodiment of FIG. 5a. The photodetector55 is divided into four photodetecting elements E, F, G and H. Thephotodetector 56 is divided into two photodetecting elements I and J bya line parallel with the tangential direction of the groove 2. Thereading system further has a plurality of adders and subtracters 57 to72, and a coefficient multiplier 77.

In order to obtain the RF signal, outputs F and H of the photodetector55 are added by the adder 68, and outputs E and G are added by the adder69. The outputs (F+H) and (E+G) are added by the adder 71. Thus, the RFsignal (E+F+G+H) is produced from a terminal 75.

In order to obtain a focus error signal, a focus error is detected by anastigmatism method using a cylindrical lens (not shown). The output(F+H) of the adder 68 and the output (E+G) of the adder 69 aresubtracted by the subtracter 72. Thus, the focus error signal(F+H)−(E+G) is produced from a terminal 76.

In order to obtain the prepit signal, the output (A+B) added by theadder 57 and the output (C+D) added by the adder 58 are subtracted bythe subtracter 64. Thus, the prepit signal (A+B)−(C+D) is produced froma terminal 73.

Describing the tracking error signal, the outputs F and G of thephotodetector 55 are added by the adder 61, and the outputs E and H areadded by the adder 62. The outputs (F+G) and (E+H) are subtracted by thesubtracter 66. The output (F+G)−(E+H) is applied to a plus terminal ofthe subtracter 70.

On the other hand, the outputs B and C of the photodetector 54 are addedby the adder 59, and the outputs A and D are added by the adder 60. Theoutputs (B+C) and (A+D) are subtracted by the subtracter 65. The output(B+C)−(A+D) is applied to one of the terminals of the adder 67.Furthermore, outputs I and J of the photodetector 56 are added by theadder 63. The output (I+J) is applied to the other terminal of the adder67. The output {(B+C)−(A+D)}+{(I+J)} of the adder 67 is applied to thecoefficient multiplier 77 where the {(B+C)−(A+D)}+{(I+J)} is multipliedby a constant K (K=0-1) for correction. The multiplied outputK[{(B+C)−(A+D)}+{(I+J)}] is applied to a minus terminal of thesubtracter 70. Thus, the tracking error signal{(F+G)−(E+H)}−{(A+D)}−K[{(B+C)−(A+D)}+{(I+J)}] is produced from aterminal 73.

In the embodiment, if the constant K is properly adjusted, noise signalcaused by the prepit 4 on the land 3 included in the original trackingerror signal (F+G)−(E+H) is reduced.

FIGS. 9a and 9 b show a third embodiment of the reading system. Thesystem is provided for reducing influence of the information written onthe groove with the prepit signal as much as possible, thereby obtainingthe prepit signal with a good C/N.

A photodetector 80 has four photodetecting elements A, B, C and D asshown in FIG. 9b. If only the prepit signal is read, a photodetectorhaving two photodetecting elements may be used.

The system further has a waveform shaping circuit 82 for producing anoise cancel signal for canceling noises which are dependent oninformation written in the groove, and subtracters 81 and 83.

Describing the operation, the photodetector 80 applies added outputs(A+B) and (C+D) to the subtracter 81 which produces the prepit signal(A+B)−(C+D). In the prepit signal (A+B)−(C+D), noises caused by theinformation written on the groove 2 are included. The prepit signal(A+B)−(C+D) is applied to the subtracter 83.

The photodetector 80 further applies the output (A+B+C+D) to thewaveform shaping 82 where the noise cancel signal is produced inaccordance with the (A+B+C+D). The noise cancel signal is applied to thesubtracter 83 where the noise cancel signal is subtracted from theprepit signal (A+B)−(C+D). Thus, the noises are canceled.

A method for producing the noise cancel signal in the waveform shapingcircuit 82 will be described with reference to FIG. 10.

It is assumed that no prepit is recorded on the land of the disc andthat the information is written on the groove. When the beam spot istracking on the groove of the disc, the added outputs (A+B) and (C+D)are shown by the waveforms (a) and (b), respectively.

On the other hand, the prepit signal (push-pull signal) is obtained bythe calculation (A+B)−(C+D). If the calculation is made, the waveform(f) is produced. The output is noise to be included in the prepit signalif prepits are recorded on the land. Accordingly, if the noises arecanceled from the prepit signal, the influence with the prepit signal isremoved.

In the embodiment, the output (RF signal) (A+B+C+D) shown by thewaveform (c) is produced by the outputs (A+B) and (C+D) shown by thewaveforms (a) and (b). The output (c) is differentiated to produce adifferential signal (d). Based on differential signal (d), the noisecancel signal having the waveform (e) is produced.

It will be seen that the noise cancel signals (e) and the noise (f) havethe same waveform. Thus, the noise cancel signal is subtracted from theprepit signal (A+B)−(C+D), which is obtained from the prepit on theland, by the subtracter 83 of FIG. 9a, so that the influence of thewritten information is removed from the prepit signal as shown by thewaveform (g), thereby obtaining the prepit signal having a good C/N.

FIGS. 11a and 11 b show a fourth embodiment of the reading system inwhich the prepit information recorded on the land is exactly read outwhile information is written in the groove of the disc withoutinterference of information signal to be written.

The reading system comprises a laser 93 for emitting the laser beam, amodulator 92 for modulating the laser beam, a prism 94, an objectivelens 95, a photodetector 90, a subtracter 91, and a waveform shapingcircuit 96.

The laser beam emitted from the laser 93 is projected on the disc 1through the prism 94 and the objective lens 95. The reflected light isdetected by the photodetector 90.

The photodetector 90 has four photodetecting elements A, B, C and D asshown in FIG. 11b.

FIG. 12 shows the power of the laser. When the information is read out,the laser power is controlled to be decreased, not to a zero level, soas to read the prepit information.

Describing the operation, the photodetector 90 applies added outputs(A+B) and (C+D) to the subtracter 91 which produces the prepit signal(A+B)−(C+D). In the signal (A+B)−(C+D), noises caused by the informationsignal to be written on the groove 2 are included.

The waveform shaping circuit 96 produces a noise cancel signal inaccordance with the modulating signal for modulating the laser power ofthe laser 93 from the modulator 92. The noise cancel signal issubtracted from the prepit signal (A+B)−(C+D) by the subtracter 91.Thus, the noises are canceled.

A method for producing the noise cancel signal in the waveform shapingcircuit will be described with reference to FIG. 13.

It is assumed that no prepit is recorded on the land. The laser beamhaving a waveform (a) for writing information on the groove is reflectedfrom the disc and received by the photodetector 90. The added outputs(A+B) and (C+D) of the photodetector 90 are shown by waveforms (b) and(c), respectively. In each of the waveforms (b) and (c), a pit writtenon the groove is represented at a position where the output is decreasedstepwise.

On the other hand, if the prepit signal calculation (A+B)−(C+D) is made,a waveform (f) is produced. The output is noise to be included in aprepit signal produced by prepit information recorded on the land.Accordingly, if the noise is canceled, the influence on the prepitsignal is removed.

In the embodiment, the writing light (a) is delayed by a time t toproduce a delay signal having the waveform (d). The delay signal (d) isshortened by the time t at the rear end so that a noise cancel signalhaving the waveform (e) is produced.

It will be seen that the noise cancel signal (e) and the noise (f) havethe same waveform. Thus, the noise cancel signal is subtracted from theprepit signal (A+B)−(C+D) obtained by recorded prepit information by thesubtracter 91 of FIG. 11a, so that the influence of the noise is removedfrom the prepit signal, thereby obtaining a prepit signal having a goodC/N.

In the embodiments, the push-pull signal (prepit signal) is formed by apair of outputs disposed on the opposite sides in the trackingdirection. Alternatively, the push-pull signal may be formed by a pairof outputs of photodetectors disposed on opposite sides of the trackdirection. In such a push-pull signal, tracking error signal and prepitsignal are included. The tracking error signal is included in thepush-pull signal as a low frequency component. The low frequency band ofthe tracking error signal is easily separated by using a filter.

It is possible to determine a left prepit or a right prepit of thegroove by a polarity.

The embodiment of the present invention can be variously modified.

In accordance with the present invention, the prepit informationincluding address information is recorded on the land as the guidingtrack. Thus, the capacity for writing the information is increased,thereby improving the available efficiency of the disc.

The prepit information is recorded on every other land. Thus, theprepits on the adjacent lands opposite to the groove are not interferedfrom each other. Thus, the prepit information with a good C/N can beobtained.

The groove is used as the information writing track, and the land isused as the guiding track. Consequently, the recorded information andthe prepit information can be ensurely and separately read out.

The prepit information for guiding track is derived based on thedifference signal between the outputs produced from the two-dividedphotodetector. The prepit information is ensurely obtained with a simplestructure.

The prepit signal and the tracking signal are obtained by the singlephotodetector. Thus, the system is reduced in size.

The prepit information for guiding track is derived based on thedifference signal between the outputs produced from the two-dividedphotodetector divided in the track direction. The tracking signal isproduced by the difference signal at a low frequency area.

In the three spot method, the prepit information and the trackinginformation are obtained by two photodetectors. Thus, the C/N is furtherimproved.

In the disc having the written information, the noise cancel signal isproduced for canceling noises caused by the written information. Thus,the prepit signal having a good C/N is obtained.

Furthermore, the noise cancel signal is produced by the modulatingsignal for modulating the laser beam. Thus, the prepit signal having agood C/N is obtained.

In the manufacturing method of the disc, since the original substrate iscut by one cutting process, the prepit on the land is not deflected,thereby manufacturing the disc of a high quality.

While the invention has been described in conjunction with preferredspecific embodiment thereof, it will be understood that this descriptionis intended to illustrate and not limit the scope of the invention,which is defined by the following claims.

What is claimed is:
 1. A reading system for reading a writable opticaldisc having an information writing track, and a guiding track forintroducing a laser beam to the information writing track, and prepitinformation including address information recorded on the guiding track,the system comprising: a photodetector having photodetecting elementsdivided at least by a dividing line optically parallel with a radialdirection of the disc for detecting reflected light of a laser beamirradiated to the information writing track, difference signal producingmeans for producing a difference signal based on a difference betweenoutputs of the divided photodetecting elements, addition signalproducing means for producing an addition signal based on an additionbetween outputs of the divided photodetecting elements, noise cancelsignal producing means for producing a noise cancel signal which cancelsnoise component of the difference signal occurring by informationrecorded on the information writing track based on the addition signal,and prepit signal producing means for producing a prepit signal based onsubtracting the noise cancel signal from the difference.
 2. The readingsystem according to claim 1, wherein the noise cancel signal producingmeans differentiates the addition signal to obtain a differentialsignal, and the noise cancel signal is produced based on thedifferential signal.
 3. A reading system for reading a writable opticaldisc having an information writing track, and a guiding track forintroducing a laser beam to the information writing track, and prepitinformation including address information recorded on the guiding track,the system comprising: a photodetector having photodetecting elementsdivided at least by a dividing line optically parallel with a radialdirection of the disc for detecting reflected light of a laser beamirradiated to the information writing track, a first subtractor forproducing a difference signal based on a difference between outputs ofthe divided photodetecting elements, an adder for producing an additionsignal based on an addition between outputs of the dividedphotodetecting elements, a wave form shaping circuit for producing anoise cancel signal which cancels noise component of the differencesignal occurring by information recorded on the information writingtrack based on the addition signal, and a second subtractor forproducing a prepit signal based on subtracting the noise cancel signalfrom the difference.
 4. The reading system according to claim 3, whereinthe wave form shaping circuit differentiates the addition signal toobtain a differential signal, and the noise cancel signal is producedbased on the differential signal.